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Update: NOT fixed in v 10.1.0 (Mar2015)


While testing the examples from this recent post, i've noticed a problem in V.10 with PlotLegends when it has to automatically generate more than 15 labels (i.e. when there are more then 15 functions to plot). There's no problem with V.9.

The problem concerns the option values : Automatic and "Expressions". For example :

Plot[Evaluate[Range[20] + x], {x, -5, 5}, PlotLegends -> "Expressions"]

enter image description here

Plot[Evaluate[Range[20] + x], {x, -5, 5}, PlotLegends -> Automatic]

enter image description here

No problem however when you specify explicitly the labels in the legend :

Plot[Evaluate[Range[20] + x], {x, -5, 5}, PlotLegends -> Range[20]]

enter image description here

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  • $\begingroup$ As I understand Mr.Wizard's answer, I would not consider this a bug but surmise it is a conscious, reasonable (undocumented) design decision by WRI. Have you reported it to WRI and did they have a response? $\endgroup$ – Michael E2 Dec 12 '14 at 11:39
  • $\begingroup$ @MichaelE2 No, I did not and do not actually plan to report that, it' s rather a minor problem, there are simple workarounds. Since it works in version 9 and there are no obvious reason why it have to be modified, I tagged it as a "bug". If not, it should be at least tagged in some way I guess ... "undocumented change" ;) I have actually bookmarked several posts which, I think, lack at least a warning tag. I've planned to post a question about that to meta because that bug tag mechanism is not actually clear for me. $\endgroup$ – SquareOne Dec 12 '14 at 13:31
  • $\begingroup$ I do think such restrictions ought to be documented (under "Possible Issues") so that users don't waste time trying to figure out why it doesn't work. And perhaps a warning message and way to override. My preference would be the V9 way plus a warning when the number of legends exceeds the number of colors. I mention this just in case someone at WRI takes a look. :) $\endgroup$ – Michael E2 Dec 12 '14 at 14:07
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    $\begingroup$ @SquareOne I removed the bugs tag according to community practice, as there is neither consensus nor confirmation (from WRI) that this is a bug. $\endgroup$ – Mr.Wizard Dec 12 '14 at 16:01
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We may observe that the automatically generated legend limits the number of legend items to the number of available colors in the given color scheme. Using this utility function:

plot[scheme_] := Plot[Evaluate[Range[20] + x], {x, -5, 5},
   PlotLegends -> "Expressions", PlotStyle -> scheme]

Observe the result for indexed color scheme #42 which has only four colors:

plot[42]

enter image description here

There are eight in #26:

plot[26]

enter image description here

As since there are 21 in #60 all your lines have a legend:

plot[60]

enter image description here

And if you specify a gradient color scheme:

Plot[Evaluate[Range[50] + x], {x, -5, 5}, PlotLegends -> "Expressions", 
 PlotStyle -> "Rainbow"]

enter image description here

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    $\begingroup$ +1, well, obviously. Otherwise they aren't distinguishable. (Yes, this was a well duh moment for me.) $\endgroup$ – rcollyer Nov 19 '14 at 13:30
  • $\begingroup$ @Kuba no it doesn't. You just need a color scheme that has enough colors, otherwise the legend algorithm says it can't distinguish them. So, they're not useful. $\endgroup$ – rcollyer Nov 19 '14 at 14:03
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    $\begingroup$ @rcollyer I know :) I should have GoldenBadge "Missunderstood guy" $\endgroup$ – Kuba Nov 19 '14 at 14:05
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    $\begingroup$ 10 colors is the most common for "Indexed" colors and 21 is the maximum number of colors. Tally[Length@ColorData[#, "ColorList"] & /@ Range@Length@ColorData["Indexed"]] gives {{15, 24}, {9, 14}, {10, 48}, {11, 7}, {8, 7}, {6, 1}, {7, 1}, {4,1}, {13, 4}, {12, 2}, {21, 2}, {14, 2}} $\endgroup$ – Bob Hanlon Nov 19 '14 at 14:46
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    $\begingroup$ @Bob A shorter way to write that: Length /@ ColorData["Indexed", "ColorList"] // Tally $\endgroup$ – Mr.Wizard Nov 19 '14 at 15:03

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